Preheat oven to 350˚F. Cream together butter and sugar in large bowl. Add eggs one at a time, followed by vanilla.
In another bowl combine flour, cocoa, baking powder, baking soda, and salt. Sift flour mixture into wet ingredients one cup at a time, alternating with milk. Fold in chopped sauerkraut.
Pour batter into 2 8” greased and lined baking rounds. Bake for 30-35 minutes or until toothpick inserted in center comes out clean. Or use one 9”x13” cake pan and bake for 35-40 minutes.
Allow to cool in pans for 30 minutes before removing to cooling rack to cool completely. Place in freezer for 10 minutes before frosting.
FOR THE FROSTING:
Beat butter and chocolate together in large bowl. Add remaining ingredients and beat until fluffy.
Level cakes with bread knife if they have become domed in baking. Place first cake on platter or plate. Frost top. Then add second cake. Do a thin layer of frosting for a crumb coat all over both cakes, then apply a thicker layer of frosting in design of your choice.